1. Field of the Invention
The present invention relates to a photometry technique in an image capturing apparatus capable of displaying, in the live view, an image which is obtained from an image sensor and is to be captured.
2. Description of the Related Art
Generally in a single-lens reflex type digital camera using an image sensor such as a CCD, a reflecting mirror is inserted in a photographing optical path extending from a photographing lens to an image sensor, so as to be directed obliquely upward in order to observe an object image. The reflecting mirror guides light from an object to an optical viewfinder. When shooting an image with the image sensor, the mirror is pivoted and retracted from the optical path to guide light from the object to the image sensor. That is, immediately before shooting an image, i.e., exposing the image sensor, the mirror is positioned on the optical path to reflect light from the object toward the viewfinder optical system. When shooting an image, the mirror is retracted from the optical path to guide light to the image sensor. Immediately after shooting an image, the mirror is returned to the optical path. A mirror of this type is called a quick return mirror.
In a single-lens reflex type digital camera having this arrangement, the quick return mirror exists on the optical path of the photographing lens in a mode other than actual photography, so no object light enters the image sensor. For this reason, the digital camera cannot perform a live view display operation of displaying an object image sensed by the image sensor in real time on a display arranged on the rear surface of the camera. The display cannot be used as an electronic viewfinder. If the electronic viewfinder can be used in addition to the optical viewfinder, this improves user friendliness. Demand has arisen for a single-lens reflex type digital camera also having the electronic viewfinder function.
To meet this demand, for example, Japanese Patent Laid-Open No. 2005-295577 proposes a digital camera having a half-mirror as a quick return mirror. In this arrangement, even when the half-mirror exists on the optical path, light enters the image sensor. The image sensor can generate an image, implementing a live view, i.e., electronic viewfinder function. Since an object image reflected by the half-mirror also enters the viewfinder optical system, the optical viewfinder can also be utilized. When shooting an image to be recorded, all light from the photographing lens can be used for photography by retracting the half-mirror from the optical path.
However, in this arrangement, light guided to the image sensor in live view display is only part of the total amount of light which have passed through the photographing lens. No high-quality live view image can be obtained by photography in a dark environment.
To attain a high-quality live view image, the quick return mirror is ideally retracted from the optical path of the photographing lens to guide all the light from the photographing lens to the image sensor.
In a single-lens reflex type digital camera, the viewfinder optical system incorporates a photometry sensor which detects the light quantity in the field. A light beam from the photographing lens is guided to the photometry sensor via the quick return mirror and pentaprism, performing field photometry. When the quick return mirror is retracted from the optical path of the photographing lens in order to use the electronic viewfinder function, the photometry sensor cannot perform field photometry.
Hence, field photometry is done using an image signal generated by the image sensor when the electronic viewfinder function is used. In this case, when natural light is measured, the time lag until the digital camera shifts from the electronic viewfinder operation to actual photography is relatively short because the image sensor always generates an image signal to perform photometry during the electronic viewfinder operation.
In flash photography, however, a preflash must be fired before actual photography. The emitted light quantity of the preflash and the light quantity distribution of reflected light from an object are detected to determine the actual emission quantity of the flash on the basis of these light quantities. Flash photography in the live view mode requires an operation to fire a preflash before shooting an image, read out an image formed by the preflash from the image sensor, and determine the actual emission quantity. This increases the time lag from pre-emission to actual emission. Since the image sensor has an array of many pixels, a long time is taken to read out signals. Photometry using the image sensor takes a longer photometry time than in that using a general photometry sensor, resulting in a long time lag. The image sensor has a narrower dynamic range than that of a general photometry sensor, and it is difficult to obtain an accurate photometric value.